Stoat (Mustela erminea) is a species of Mustelidae native to Eurasia and North America and were introduced into New Zealand to control rabbits and hares, but are now a major threat to the native bird population. The natural range of the stoat is limited to parts of the Northern Hemisphere. Immediately prior to human settlement, New Zealand did not have any land-based mammals apart from bats, but Polynesian and European settlers introduced a wide variety of animals.

Introductions of stoats: The rabbit was introduced as a food and game animal by European settlers and by the 1870s, it was becoming a serious threat to the newly developed farming economy. Farmers began demanding the introduction of mustelids (including stoats) to control the rabbit plague. Warnings about the dangers to birdlife from stoats were given by scientists in New Zealand and Britain, including the New Zealand ornithologist Walter Buller. The warnings were ignored and stoats began to be introduced from Britain in the 1880s, then, within six years, drastic declines in bird populations were noticed.In December 2010, a stoat was seen on what was thought to be the stoat-free Kapiti Island, and by August the next year, DOC had managed to kill three. It is thought that they could not have swum the five-kilometre stretch of open sea from the Kapiti Coast.

The threat to bird life: New Zealand has a high proportion of ground-nesting and flightless birds, due to the long geographical isolation and a lack of mammal predators. The native birds have evolved to fill the niche that is otherwise filled with mammals. Stoats are the greatest threat to these ground-nesting and hole-nesting birds since they have very little means of escaping predation. In addition to birds, stoats eat insects and mice. During "beech masts" when the southern beeches (Nothofagus species) produce a far greater than normal amount of seed, the stoat population undergoes changes in predation behaviour. With high beech-seed numbers, rats and mice become more plentiful; this increase in prey encourages stoat breeding. The higher stoat numbers reduce the rodent population and the stoats then prey on birds. For instance, the endangered takahe's wild population dropped by a third between 2006 and 2007, after a stoat plague triggered by the 2005–06 mast wiped out more than half the takahe in untrapped areas.

Stoat control measures: Stoats are difficult to control since they are bait-shy, trap-wary and have a high fecundity. In some areas where there are populations of endangered birds, a programme of stoat-trapping has been implemented. The most common method of trapping is to use a stoat tunnel - a wooden box with a small entrance at one end to allow the stoat to enter. The bait is an egg and a trap is placed in the tunnel to kill the stoat. Mainland Islands, protected areas that have intensive control of introduced pests, have stoat trapping on their perimeter. Predator-proof fences are used to keep stoats out of protected areas with the use of fine wire mesh netting. Methods of restricting stoat breeding are also being investigated.

Stoat legislation: Although stoats were recognised as a potential pest before being introduced into New Zealand, they were given protection as late as 1936. As a means of preventing a loss of biodiversity, there are no penalties for introducing stoats into protected areas. It was named one of the world's top 100 "worst invasive species" by the IUCN Species Survival Commission's Invasive Species Specialist Group.Thanks to Wikipedia for the above text.

Stoats vary in colour from reddish-brown to a greyish brown. a white or cream coloured underbelly, and has a long tail relative to weasels, with a distinctive and obvious bushy black tip. An adult male can measure 390 mm from nose to tip of the tail. Stoats can be confused with weasels but adult stoats are longer and heavier than adult weasels and display a straight line along their sides where the brown fur meets the pale belly fur.

The stoat below was trapped next to the Egmont Reserve South Taranaki.